Photoneutron Source Carachterization and Neutron Simulations

Alan Robinson

Sept 24, 2015Low Energy Calibration Workshop

Slide 2/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Photoneutron Sources● Portable

● Monochromatic

● Low-energy

● 105 – 106 photons/neutron

Slide 3/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Photoneutron Sources

Source Half-life Photons Neutron Energy226Ra 1600 y 1764 keV (15.3 %)

2204 keV (4.92 %)others (9.69 %)

89 keV479 keV

124Sb 60.2 d 1691 keV (47.6 %)2091 keV (5.49 %)

others (0.52 %)

23.5 keV379 keV

88Y 106.6 d 1836 keV (99.9 %)2734 keV (0.71 %)

152 keV950 keV

207Bi 31.6 y 1770 keV (6.87 %) 94 keV

Radioisotope / beryllium sources

Slide 4/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Photoneutron Calibrations● Need

►Well defined energy spectrum

►Absolute recoil rate

● Measure source yield

● Minimize moderator

● Understand neutron recoil response.

Slide 5/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Photoneutron History● The NIST calibrated neutron source is a 1 Ci

226Ra/Be photoneutron source.

Slide 6/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Photoneutron History● Measurements of 9Be(γ,n) cross-section

►1940-60's & 1982: Radioisotopes.

►1960-70's: Bremstrallung photons, charged particle scattering.

►2000 & 2012: Inverse compton photons.

Slide 7/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations

9Be(,n) Cross-sectionA selection of existing measurements

Slide 8/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations

9Be(,n) Cross-section● Reanalyzed radioisotope measurements and

corrected for

►Branching ratios

►Detector efficiencies (using modern MCNP simulations)

● Fitted to Breit-Wigner resonance parameters.

En = 1738.8 keV, Γn = 268.7 keV, Γγ = 0.7715 eV

Slide 9/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations

9Be(,n) Cross-section

Slide 10/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Source Yield● Calculated from known

cross-sections

►0.659 mb at 1836 keV

►1.397 mb at 1691 keV

►955 mb at 1770 keV

● Directly measured using He-3

Slide 11/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Source Design● Photoneutron emission from 9Be

radioisotope sources is isotropic in the rest frame.

►Energy-angle dependence in the lab frame.▻±6% spread in Sb/Be neutron energy.

● Photoneutron yield and energy spectrum depend on the source geometry.

►Diminishing returns on yield of unmoderated neutrons for Be or BeO thickness >1cm.

Slide 12/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Source Design

Low yield uncertaintyLarger spread of

neutron energies

Large yield sensitivity to BeO – 88Y distance.

Slide 13/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Moderation● Energy degradation in BeO may be

subdominant to other moderators.

Slide 14/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Source Design● If using a lead shield

►Surround the source with lead on all sides so that lead is a diffuser instead of a reflector.

Slide 15/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Neutron Moderation● Libraries for both MCNP and Geant4

incorrectly describe the angular distribution from neutron scattering for most isotopes.

►Problem not applicable to oxygen and lighter.

►New libraries for F, Si, Ar, Cr, Fe, and Pb are available. PRC 89 032801 (2014).

Slide 16/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Neutron Moderation

Slide 17/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Neutron Recoils

Slide 18/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Neutron Recoils● Recoil spectrum uncertainties

►Cross-section uncertainty

►Resonance parameter uncertainty

►Unresolved resonances

Extra slides

Slide 20/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Neutron Elastic Scattering● Resonances from compound states

● Continuum from potential scattering

Slide 21/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Breit-Wigner Resonances

● Parameterized by

►Resonance widths

►Neutron Energy

►Spins

►Parity

● Interference handled by R-matrix formalism

Slide 22/18

LowECalSept 24, 2015

Photoneutron Sources and Simulations Recalculating d/d● Dipole term of scattering angle distribution

vs. energy